SiMo DUCTILE IRON CASTINGS IN GAS TURBINE APPLICATIONS

ABSTRACT

A cast article of a ductile iron wherein the ductile iron includes carbon from about 2.8 to 3.7 w/o, silicon from about 3.0 to 3.5 w/o, molybdenum from 0.8 to 1.5 w/o, magnesium from about 0.025 to 0.60 w/o, sulfur less than 0.01 w/o and nickel from about 0.0 to 1.3 w/o, the remaining content being iron is provided. The cast article is suitable for a gas turbine casing. A method of manufacturing a cast article is also provided.

BACKGROUND OF THE INVENTION

The invention relates to ductile iron for use in gas turbines thatprovides cost benefits and improved supplier choices.

Currently, gas turbine casings operating at elevated temperatures(greater than 370° C.) are restricted to alloy steel castings orfabrications. Gas turbines must endure unsteady operation to cover peakloads. This places thermal and mechanical stresses on the gas turbinecomponents. Therefore, gas turbine casings must be able to withstandhigh temperature environments and repeated temperature cycling. Thestrength of the gas turbine casing material at high temperatures must behigh. Presently, alloy steel castings for gas turbine casings meet theserequirements; however, gas turbine casings of alloy steel are expensiveto manufacture and there are a limited number of suppliers.

Traditional ferritic ductile irons are less costly than alloy steels buttypically, have inadequate combination of properties, precluding theiruse in advanced gas turbine compressor discharge and turbine shellcasings. Irons with higher silicon and molybdenum contents have founduse in certain automotive applications, typically exhaust manifolds.These irons are referred to as SiMo irons. However, these irons aregenerally brittle at cold temperatures making them likely to crack. Inaddition, these materials do not possess the requisite toughness atelevated temperatures. Examples of such materials are found in US Pub.2008/0092995, WO 2006/121826, U.S. Pat. No. 6,508,981 and EP 1724370 A1.

With increasing casing size it becomes more costly to manufacture gasturbine casings from steel castings. In addition, the current supplybase to produce such large steel castings is small.

SUMMARY OF THE INVENTION

Embodiments of the invention include a ductile iron gas turbine casingwherein the ductile iron includes carbon from about 2.8 to 3.7 w/o,silicon from about 3.0 to 3.5 w/o, molybdenum from about 0.8 to 1.5 w/o,magnesium from about 0.025 to 0.60 w/o, sulfur less than about 0.01 w/oand nickel from about 0.0 to 1.3 w/o, the remaining content being iron.

Embodiments of the present invention also include a ductile iron gasturbine casing wherein the ductile iron includes carbon from about 2.8to 3.7 w/o, silicon from about 3.0 to 3.5 w/o, molybdenum from about 0.8to 1.5 w/o, magnesium from about 0.025 to 0.60 w/o, sulfur less than0.01 w/o, nickel from about 0.0 to 1.3 w/o, phosphorous less than 0.05w/o, titanium less than 0.05 w/o, vanadium less than about 0.05 w/o, tinless than 0.05 w/o, aluminum less than about 0.10 w/o, copper less thanabout 0.10 w/o, chromium less than about 0.10 w/o and manganese at lessthan about 0.15 w/o, the remaining content being iron.

Embodiments of the present invention also include a method ofmanufacturing a component. The method includes melting ductile iron thatincludes carbon, silicon, magnesium, sulfur and nickel, the remainingcontent being iron to form a melt. Inoculants and treatment alloys areadded to the melt. Molybdenum is added to the melt. The melt is cast toform the component. The component includes carbon from about 2.8 to 3.7weight percent, silicon from about 3.0 to 3.5 weight percent, molybdenumfrom about 0.8 to 1.5 weight percent, magnesium from about 0.025 to 0.60weight percent, sulfur less than about 0.01 weight percent and nickelfrom about 0.0 to 1.3 weight percent, the remaining content being iron

The above described and other features are exemplified by the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various embodiments of the invention, in which:

FIG. 1 shows a block diagram of an illustrative method for implementingone embodiment of the invention.

DETAILED DESCRIPTION

High temperature strength, fatigue and creep behavior of ductile ironcan be improved with large alloy additions of silicon and molybdenum.These irons are commonly classified as SiMo irons and have been usedextensively in automotive applications such as turbocharger housings andexhaust manifolds.

Ductile irons typically fail to meet design requirements for hightemperature gas turbine casing applications such as compressor dischargecasings or turbine shells. Generally alloyed steels are used for gasturbine casings.

Ductile iron with improved high-temperature performance overconventional ferritic ductile iron by alloy additions of molybdenum andsilicon is presented. The silicon and molybdenum additions are balancedto achieve adequate high temperature properties while retainingsufficient low temperature toughness.

Standard silicon levels in heavy-section ductile iron are typicallybetween 2.0 and 2.2 weight percent (w/o). It has been found thatincreasing silicon content to between 2.8 and 3.5 w/o substantiallyincreases tensile strength between room temperature and about 400° C.Isothermal creep performance for a 0.5 w/o molybdenum ductile iron issubstantially better than conventional ductile iron. High temperaturestrength is an indication of creep resistance. Improvements in creepresistance performance is maximized as the Mo content is increased from0.8 to 1.5 weight percent (w/o). By using Si and Mo in the weightpercentages shown a ductile iron with properties suitable for gasturbine casings is provided.

The ductile iron used in embodiments of the present invention includescarbon from about 2.8 to 3.7 w/o, silicon from about 3.0 to 3.5 w/o,molybdenum from about 0.8 to 1.5 w/o, nickel from about 0.0 to 1.3 w/o,the remaining content being iron. These four components are critical toproviding a ductile iron that meets the requirements for gas turbinecasings. At elevated temperatures, tensile strength as well as low cyclefatigue (LCF) capability is determined by the occurrence of elevatedtemperature brittleness. Magnesium must be kept from 0.025 to 0.60 w/oto provide a SiMo ductile iron with the proper characteristics withsulfur less than 0.01 w/o. Magnesium loadings outside this range produceiron that generally has inadequate mechanical behavior. In addition tothe components listed above, minor amounts of the following componentsare allowable. Phosphorous at less than 0.05 w/o, titanium at less than0.05 w/o, vanadium at less than 0.05 w/o, tin at less than 0.05 w/o,aluminum at less than 0.10 w/o, copper at less than 0.10 w/o, chromiumat less than 0.10 w/o and manganese at less than less than 0.15 w/o. Inone embodiment, the tungsten content of the SiMo ductile iron is lessthan 0.05 w/o.

Molybdenum-rich eutectic phases can lead to poor mechanical properties,specifically elongation and toughness. The strong partitioning ofmolybdenum to cell boundaries in the form of eutectic, intermetallic ormetallic carbide phases is unavoidable. However, these phases can bereduced to acceptable levels by proper inoculation and chilling as wellas implementing of other standard foundry practices.

A method of producing SiMo ductile iron is shown in FIG. 1. In step 10,the iron and other components are melted. Specified product chemistry isnot identical to melt chemistry. As there are losses associated with theinitial liquid melt, the final melt chemistry is different than theinitial melt chemistry. In step 11, standard inoculants and treatmentalloys are added to the melt. A molybdenum alloy is added to the melt instep 12. The melt is cast to form the part in step 13. Higher drosslevels are associated with SiMo iron chemistry so these levels need tobe accounted for in the foundry. Additionally, higher shrinkage andreduced feeding characteristics of the parts are typical. A heattreatment or ferritizing anneal in step 14 is generally required toimprove toughness and prevent cracking during handling at the foundry.

A typical heat treatment or ferritizing anneal process for the castmaterial is as follows. Hold cast part at 900° C. for at least 7 hours.Allow part cool to 720° C. and hold for at least 2 hours. Allow partcool to 690° C. and hold for at least 8 hours.

Rather than a ferritizing anneal process, a stress relief anneal can beperformed on cast parts. The stress relief anneal is from about 650 to750° C. for 1 hour per inch thickness of the section with the greatestthickness.

Inoculation is required to promote the formation of graphite instead ofmetastable carbide. Inoculants provide heterogeneous nucleation sites(seeds) for graphite to form. The primary component of the inoculant issilicon. Foundry grade ferrosilicon (75 w/o Si) is often used forinoculation. Typical commercial inoculants contain high levels ofsilicon plus various levels of calcium, germanium, strontium, and rareearth elements (cerium is most common due to other beneficialcharacteristics in heavy section iron). Inoculants are often addedmultiple times in the production of large ductile iron castings.Suitable inoculants are available from many sources.

Standard treatment alloys are added to the melt in step 11 of FIG. 1.Treatment (sometimes referred to as modification) is necessary to forcethe formation of graphite spheroids instead of flakes. Treatment can bein the form of nearly pure Mg in powder form (George Fischer converter)or most cases in the form of a Mg-bearing alloy (often with Nickel).

Shrinkage occurs during solidification. Chilling (strategic placement oflarge cast iron blocks to remove heat) is used to promote directionalsolidification to limit macroscopic shrinkage porosity in critical areasof the casting. The size, type, number and placement of these chillsbecomes more important in SiMo irons due to reduced feeding andshrinkage levels associated with these irons.

Risers (or feeders) are needed to supply molten metal to prevent largeshrinkage porosity in critical locations. These risers are often placedin regions susceptible to shrinkage (hard to feed thick-to-thin geometrytransitions, for example). General foundry practice requires thedistance between risers to decrease as the castability decreases.Additionally, larger risers and riser necks are often used as thecastability decreases. Adjustments in pouring temperature are alsocommon as the castability of the alloy decreases.

The terms “first,” “second,” and the like, herein do not denote anyorder, quantity, or importance, but rather are used to distinguish oneelement from another, and the terms “a” and “an” herein do not denote alimitation of quantity, but rather denote the presence of at least oneof the referenced item. The modifier “about” used in connection with aquantity is inclusive of the stated value and has the meaning dictatedby the context, (e.g., includes the degree of error associated withmeasurement of the particular quantity). The suffix “(s)” as used hereinis intended to include both the singular and the plural of the term thatit modifies, thereby including one or more of that term (e.g., themetal(s) includes one or more metals). Ranges disclosed herein areinclusive and independently combinable (e.g., ranges of “up to about 25w/o, or, more specifically, about 5 w/o to about 20 w/o”, are inclusiveof the endpoints and all intermediate values of the ranges of “about 5w/o to about 25 w/o,” etc.).

While various embodiments are described herein, it will be appreciatedfrom the specification that various combinations of elements, variationsor improvements therein may be made by those skilled in the art, and arewithin the scope of the invention. In addition, many modifications maybe made to adapt a particular situation or material to the teachings ofthe invention without departing from essential scope thereof. Therefore,it is intended that the invention not be limited to the particularembodiment disclosed as the best mode contemplated for carrying out thisinvention, but that the invention will include all embodiments fallingwithin the scope of the appended claims.

1. A gas turbine casing comprising: a cast ductile iron wherein theductile iron comprises: carbon from about 2.8 to 3.7 weight percent,silicon from about 3.0 to 3.5 weight percent, molybdenum from about 0.8to 1.5 weight percent, magnesium from about 0.025 to 0.60 weightpercent, sulfur less than about 0.01 weight percent and nickel fromabout 0.0 to 1.3 weight percent, the remaining content being iron. 2.The gas turbine casing of claim 1, further comprising phosphorous,titanium, vanadium and tin each at weight percent of less than about0.05.
 3. The gas turbine casing of claim 1, further comprising aluminum,copper and chromium each at weight percent of less than about 0.1. 4.The gas turbine casing of claim 1, further comprising manganese atweight percent of less than about 0.15.
 5. The gas turbine casing ofclaim 1 further comprising tungsten at a weight percent of less thanabout 0.05.
 6. A gas turbine casing comprising: a cast ductile ironwherein the ductile iron comprises: carbon from about 2.8 to 3.7 weightpercent, silicon from about 3.0 to 3.5 weight percent, molybdenum fromabout 0.8 to 1.5 weight percent, magnesium from about 0.025 to 0.60weight percent, sulfur less than 0.01 weight percent, nickel from about0.0 to 1.3 weight percent, phosphorous less than about 0.05 weightpercent, titanium less than about 0.05 weight percent, vanadium lessthan about 0.05 weight percent, tin less than about 0.05 weight percent,aluminum less than about 0.10 weight percent, copper less than about0.10 weight percent, chromium less than about 0.10 weight percent,manganese less than about 0.15 weight percent, tungsten less than about0.05 weight percent, the remaining content being iron.
 7. A method ofmanufacturing a component, the method comprising: melting ductile ironcomprising carbon, magnesium, sulfur and nickel, the remaining contentbeing iron to form a melt; adding inoculants and treatment alloys to themelt; adding molybdenum to the melt; and casting the melt to form thecomponent wherein the component comprises carbon from about 2.8 to 3.7weight percent, silicon from about 3.0 to 3.5 weight percent, molybdenumfrom about 0.8 to 1.5 weight percent, magnesium from about 0.025 to 0.60weight percent, sulfur less than 0.01 weight percent and nickel fromabout 0.0 to 1.3 weight percent, the remaining content being iron. 8.The method of claim 7 further comprising: annealing the component aftercasting the melt.
 9. The method of claim 8, wherein the annealingcomprises: holding the component at 900° C. for at least 7 hours;cooling the component to 720° C. and holding for at least 2 hours;cooling the component to 690° C. and holding for at least 8 hours. 10.The method of claim 7 wherein the component comprises a gas turbinecasing.
 11. The method of claim 7, wherein the component comprisesphosphorous, titanium, vanadium, and tin each at weight percent of lessthan about 0.05.
 12. The method of claim 7, wherein the componentcomprises aluminum, copper and chromium each at weight percent of lessthan about 0.1.
 13. The method of claim 7, wherein the componentcomprises manganese at weight percent of less than about 0.15.
 14. Themethod of claim 7 wherein the component comprises tungsten at a weightpercent of less than about 0.05.
 15. The method of claim 7 wherein theinoculants comprise ferrosilicon.
 16. The method of claim 15 wherein theinoculants further comprise calcium, germanium, strontium and rare earthelements.
 17. The method of claim 7 wherein the treatment alloyscomprises magnesium and nickel.
 18. The method of claim 7 furthercomprising; stress relief annealing at about 650 to 750° C. for 1 hourper inch thickness of the component.
 19. The method of claim 7 whereinthe casting comprises placement of large cast blocks to remove heat.